1. Field of the Invention
The present invention relates to a braking control apparatus for hydraulically braking a slewing structure in a slewing type working machine such as a crane.
2. Description of the Related Art
There has been conventionally known an apparatus for braking a slewing motion of a slewing structure in a working machine such as a crane, as described in, for example, Japanese Unexamined Patent Application Publication No. 2001-72381. This apparatus comprises a hydraulic motor for driving to slew, a hydraulic pump for supply to the hydraulic motor, a pilot controlled control valve positioned between the hydraulic motor and the hydraulic pump, a remote control valve for generating a pilot pressure according to an operation of a control lever, and a pair of brake valves for braking the slewing.
The control valve has a neutral position for intercepting a supply line connected to two ports of the hydraulic motor (for shutting off the hydraulic motor from the hydraulic pump) and pilot ports for receiving a pilot pressure, which produces an opening action of the control valve in a stroke corresponding to the pilot pressure. As the opening of the control valve is thus increased, the hydraulic oil discharged from the hydraulic pump is led to the hydraulic motor in a direction and a flow rate corresponding to the stroke, rotating the motor.
Each of the brake valves, which is formed of a pilot controlled valuable relief valve, is positioned between the supply line and a tank. The brake valve is opened by the sudden return of the control lever from its operation position to its neutral position, thereby preventing the hydraulic motor and the slewing structure from being given a strong shock.
This apparatus, however, involves a difficult braking operation which can be made only by high-skilled operators. Specifically, the apparatus requires a sensitive return operation of the control lever for controlling a degree of slewing braking: the relation between the operation of the control lever and the actual braking action cannot be grasped without rich experience.
On the other hand, there is known also an apparatus having a mechanical brake operated in synchronization with a depressing operation of a brake pedal, for performing a braking action without the above operation on the control lever (for example, as described in Japanese Unexamined Patent Application Publication No. 2006-90080); however, the mechanical brake includes a brake band and the like which wears so seriously as to have to be replaced in a short maintenance period. Moreover, the performance of the mechanical brake is tend to be degraded due to a penetration of earth and sand or water, requiring a special structure for protecting itself.